It is common in the art to utilize an antenna array comprised of a plurality of antenna elements in order to illuminate a selected area with a signal or signals. Often such an array is used in combination with beam forming techniques, such as phase shifting the signal associated with particular antenna elements of the array, such that the signals from the excited elements combine to form a desired beam, or radiation pattern, having a predetermined shape and/or direction. Similarly, the antenna elements of such an array may each provide illumination of a select area, or sector, of the array's total radiation pattern, thus providing designated signals to a selected area.
Irrespective of their use to provide adaptive techniques or to provide signal coverage within a sector, these antenna elements must typically be provided signals having some component unique to the particular antenna element for which it is to excite. For example, where the array utilizes beam steering, adjacent antenna elements may each be provided with a signal having common information content but phase shifted in order to form a desired composite radiated signal. Likewise, where the array utilizes the individual antenna elements to provide signals within selected sectors, only those antenna elements associated with particular sectors need be provided with a signal (i.e., the power component of the signal at an antenna element associated with an unused sector may be zero).
Therefore, it is often desirable to provide signal input paths sufficient in number to result in the controllable excitation of the antenna columns as described above. For example, where sixteen antenna elements are to be utilized in an array, sixteen signal input paths, each associated with a particular antenna element, may be utilized.
In addition to the ability to apply a selected signal to particular ones of the antenna elements, in order to provide a signal of sufficient amplitude, it is often desirable to provide amplification in each of the signal input paths. One method of providing such signal amplification is to provide linear power amplifiers (LPA) in signal paths directly coupled to each antenna element. In our example, having sixteen signal input paths associated with sixteen antenna elements, this method requires sixteen LPAs, or one LPA coupled to each antenna element.
However, LPAs are expensive and often cumbersome to implement. For example, they are relatively heavy and therefore often difficult to deploy in a typical antenna system environment. Similarly, the LPAs are active components consuming power and producing heat as a by-product and are susceptible to failure. Therefore, it is desirable to both reduce the number of LPAs necessary for any particular antenna configuration, as well as to provide for signal transmission to any antenna element even in the case of an inoperative LPA.
An alternative method of providing amplified signals to the antenna elements of an array uses a back to back Butler matrix combination having sixteen LPAs disposed between a Butler matrix and an inverse Butler matrix to provide a distributed amplifier arrangement. The advantage of this arrangement is that a Butler matrix takes a signal input at any of the matrix's inputs and effectively provides a Fourier transform of the signal. This results in an input signal, provided to a single input of the Butler matrix, appearing at each of the matrix's outputs as a linear phase progression (i.e., the input signal is dissected into spectral components each appearing at a different Butler matrix output). By amplifying each of these spectral component signals, and applying the result to an inverse Butler matrix, an amplified version of the original signal, including all of its spectral components, may be had.
Similarly, a back to back hybrid matrix combination having sixteen LPAs disposed between a hybrid matrix and an inverse hybrid matrix providing a distributed amplifier arrangement may be used. Although not a Fourier transform, the input signal is nonetheless provided to each of the matrix's outputs (here, because of the phase shift relationship of the hybrid splitters used, the signals appearing at the hybrid matrix's output are component signals having a phase difference equal to that of the hybrid splitter as between adjacent signal components). As such, by amplifying each of these power component signals, and applying the result to an inverse hybrid matrix, an amplified version of the original signal results. The use of such a hybrid matrix combination is often preferable to the aforementioned Butler matrix combination as the cost associated with a hybrid matrix is considerably lower than that of the Butler matrix.
One advantage of the above described back to back Butler and hybrid matrix arrangements is that, by definition, the arrangements provide distributed amplification of any input signal (i.e., a signal input at any single input signal path is evenly distributed across all LPAs). As such, the arrangements provide advantages of distributed amplification, such as amplifier operation in a more linear range as well as fault tolerance for an inoperative LPA.
In contrast to this matrix arrangement, the directly coupled LPA method of providing amplification, described above, must supply all gain associated with any signal through a single amplifier, or series of amplifiers having a single signal path. It becomes readily apparent that such an arrangement provides no fault tolerance for an LPA failure. As the LPAs are simply disposed directly between the signal input path and an associated antenna element, if an LPA fails then the signal path is disrupted and the associated antenna element is no longer provided a signal.
However, in the matrix arrangements, if one or even a number of the LPAs malfunction it is still conceivable that performance may be had from all of the antenna elements. This is so because the first Butler or hybrid matrix distributes components of the input signal along its output. These components are then each amplified by the LPAs and subsequently recombined by the inverse Butler or hybrid matrix to reconstruct the original signal. Accordingly, if a few of the signal components are missing, such as due to failure of one or more of the LPAs, the inverse matrix can still reconstruct the signal fairly accurately. The reconstructed signal is not an exact reproduction of the original signal, but is accurate enough to provide a signal to the right antenna element.
However, it shall be appreciated that, although providing a desired advantage of fault tolerance, it is very difficult, if not impossible, to configure either the back to back Butler or hybrid matrix to provide for input and output signal paths of numbers different than those of the power two (i.e., typical back to back Butler and hybrid matrixes are limited to inputs and outputs numbering 4, 8, 16, . . . 2.sup.n). Because of this limitation, use of a number of antenna elements differing from a power of two requires a Butler or hybrid matrix having more input and output signal paths than actually utilized. For example, an antenna array having only twelve antenna elements must use a 16.times.16 Butler matrix while only utilizing twelve of the input and twelve of the output paths.
Although the above arrangement will satisfactorily provide the advantages of distributed amplification, it should be appreciated that, because the matrix arrangement distributes any input signal as signal components among all its outputs, a total of sixteen LPAs are required. As described above, it is desirable to reduce the number of LPAs necessary to provide the desired signal to an antenna array. However, where the number of antenna elements is other than a power of two, a distributed amplifier utilizing a back to back matrix arrangement actually requires more LPAs than there are input signals or antenna elements.
Therefore, a need exists in the art for a system and method by which various numbers of input signals associated with an antenna array or other system may be amplified while providing the advantages of a distributed amplifier.
A further need exists in the art for a system and method providing for the distributed amplification of numbers of signals other than powers of two while requiring a minimum number of amplifiers.
A still further need exists in the art for providing suitable fault tolerance in the amplification of a number of signals.